The printing original plates for lithography which are used mainly in the filed of small-scale printing include (1) a direct draw type original plate having a hydrophilic image-receiving layer on a waterproof support, (2) an original plate having on a waterproof support an (lipophilic) image-receiving layer comprising zinc oxide, which is converted into a printing plate by undergoing direct draw plate-making and further desensitizing treatment with a desensitizing treatment solution for the non-image area, (3) an original plate of an electrophotographic light-sensitive material having on a waterproof support a photoconductive layer comprising photoconductive zinc oxide, which is converted into a printing plate by undergoing an image forming operation and further a desensitizing treatment with a desensitizing treatment solution for the non-image area, and (4) an original plate utilizing a silver-halide photographic material which has a silver halide emulsion layer on a waterproof support.
With the development of office appliances and the expansion of office automation in recent years, it has been desired in the field of graphic arts to adopt an offset lithographic printing system wherein the lithographic printing original plate of direct draw type (the foregoing type (1)) is made directly into a printing plate using some of various platemaking (image forming) means, e.g., an electrophotographic printer, a heat-sensitive transfer printer or an ink jet printer without undergoing any special treatments for conversion into a printing plate.
In particular, nowadays the ink jet recording method is spreading rapidly because it enables noiseless and high-speed printing.
With respect to the ink jet recording method, various ink jet recording systems, e.g., the so-called electric field control system which jets out ink by utilizing induced electrostatic force, the so-called drop-on-demand system (pressure pulse system) which jets out ink by utilizing oscillating pressure of piezo elements, and the so-called bubble (thermal) jet system which jets out ink by utilizing the pressure of bubbles produced and grown by means of high thermal energy have been proposed, and these systems can provide images of high accuracy.
Further, another direct platemaking method of the printing plate wherein an electrophotographic printer is utilized has been proposed. More specifically, this method is adopted in the electronic editorial system wherein the input, correction, editing, layout and page make-up are performed by a continuous computer operation and the thus processed image information is instantly transmitted into terminal plotters in distant places via high-speed communication network or a communications satellite. In this system, a digital signal input adaptable electrophotographic printer is used as a terminal plotter, and printing plates are made directly from the output of the printer.
In a conventional lithographic printing original plate of direct draw type, the support, such as paper, has on the both surface side an image-receiving layer which is a surface layer provided via an interlayer or an under(coat) layer. The under layer and the interlayer are each constituted of a water-soluble resin, such as PVA or starch, a water-dispersible resin, such as a synthetic resin emulsion, and a pigment. The image-receiving layer comprises an inorganic pigment, a water-soluble resin and a water resisting agent.
Examples of a hitherto used inorganic pigment include kaolin, clay, talc, calcium carbonate, silica, titanium oxide, zinc oxide, barium sulfate and alumina.
Examples of a hitherto used water-soluble resin include polyvinyl alcohol (PVA), modified PVA such as carboxylated PVA, starch and derivatives thereof, cellulose derivatives such as carboxymethyl cellulose and hydroxyethyl cellulose, casein, gelatin, polyvinyl pyrrolidone, vinyl acetate-crotonic acid copolymer, and styrene-maleic acid copolymer.
Examples of a hitherto used water resisting agent include glyoxal, initial condensates of aminoplasts such as melamine-formaldehyde resin and urea-formaldehyde resin, modified polyamide resins such as methylolated polyamide resin, polyamide-polyamine-epichlorohydrin adduct, polyamide epichlorohydrin resin, and modified polyamidepolyimide resin.
In addition to the above ingredients, it is also known that a cross-linking catalyst such as ammonium chloride or a silane coupling agent can also be combination-used.
With respect to the ink used for forming images on a conventional lithographic printing original plate of direct draw type in accordance with an ink jet recording system, water-based ink which uses water as the main solvent and oil-based ink which uses an organic solvent as the main solvent are generally used.
However, the water-based ink has drawbacks of blurring the images on the plate and causing a decrease of drawing speed due to slow drying. With the intention of mitigating such drawbacks, the method of using oil-based ink using a nonaqueous solvent as dispersing medium is disclosed in JP-A-54-117203 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Even when such a method is adopted, however, image blur is actually observed on a plate-made image obtained, and further blur is generated upon printing. In addition, the number of printed matter producible with the printing plate is of the order of several hundreds at the most, so it is far below the required level. Moreover, the foregoing ink has a problem of being apt to clog up a nozzle for jetting out so fine ink drops as to form plate-made images of high resolution.
In the ink jet recording system, the ink is generally passed through a filter and then jetted out from a nozzle. Thus, this system tends to cause ink jet troubles attributable to various factors such that the nozzle is liable to be clogged up, the filter is liable to be stuffed up, the ink-fluidity changes with the lapse of time, and so on.
Such ink jet troubles are caused by not only water-based ink compositions but also oil-based ink compositions. For improving the ink jet troubles, various proposals have been submitted. For instance, for preventing those ink jet troubles in the case of using an oil-based ink composition in the ink jet recording system of electric field control type, JP-A-49-50935 proposes controlling the viscosity and the specific resistance of the ink composition, and JP-A-53-29808 proposes controlling the specific resistance and the dielectric constant of a solvent used for the ink composition.
Further, as attempts to prevent clogging of the nozzle due to oil-based ink for a printer in the ink jet recording system, the methods of improving the dispersion stability of pigment particles (described in JP-A-4-25573, JP-A-5-25413, and JP-A-5-65443), the methods of incorporating particular compounds in ink compositions (described in JP-A-3-79677, JP-A-3-64377, JP-A-4-202386, JP-A-7-109431) have been proposed.
However, even if any of the ink compositions improved by those methods is used for image formation on the printing original plate, the images formed suffer from insufficiency of strength upon printing, so the resulting lithographic printing plate cannot have a satisfactory press life.
Furthermore, for improving the printing durability of conventional printing plates made in the aforementioned manners, if the hydrophobicities of those printing plates are enhanced by adding a water resisting agent in a large amount or by using a hydrophobic resin, the scum due to the lowering of water wettability (affinities of the plates for water) is generated although the press life is improved; while the enhancement of water wettability (affinities of the plates for water) results in the lowering of water resistance to cause deterioration of press life.
In particular, when those printing plates are used under a temperature condition of 30.degree. C. or more, they have a defect that the surface layer thereof are dissolved in a fountain solution used for offset printing to result in deterioration of press life and generation of scum. Moreover, since the images are drawn directly on the image-receiving layer of the printing original plate with oil-based ink in the case of direct draw lithography, poor adhesion of the oil-based ink to the image-receiving layer causes the ink to come off the image area during printing operations, thereby deteriorating the press life even if the non-image area does not generate scum because of sufficient water wettability. This problem does not yet come to a satisfactory solution.
On the other hand, in the case of adopting the platemaking method wherein images are formed on the printing original plate having a zinc oxide-containing image-receiving layer by the use of a heat-sensitive transfer recording system, an ink jet recording system or an electrophotographic recording system, and then the non-image area is treated with a desensitizing solution, the image of plate-made printing plate and printed matter have good quality and a great number of printed matters having good quality can be provided. However, this method has the complication in wet processing. For example, it is essential for the method to use a desensitizing solution in the course of platemaking and a fountain solution containing the same desensitizing component as the desensitizing solution at the time of printing. In addition, it occurs, though depends on the printing ink used, that the foregoing component in the fountain solution used has interaction with some component in the printing ink to result in staining the printed matter. Thus, this method has a problem of being unsuitable for the color printing with a wide variety of printing inks.
In the field of digital adaptable electrophotographic printers, remarkable technical improvements have been made lately. For instance, the reproduction of high resolution image have been achieved by an electrophotographic printer using fine dry toner having a particle size of 6 to 8 .mu.m, and the reproduction of highly precise images with a high reproducibility have been achieved by an electrophotographic printer using liquid toner.
In drawing images on a printing original plate of direct draw type by image transfer using, e.g., a laser printer of such a system as mentioned above, therefore, it is required that both prevention of scumming in the non-image area after transfer and high image reproducibility in the image area be achieved to provide printed matters having clear images and no scumming, in great numbers. Further, it is desired that printed matter having a wide variety of color images be easily obtained.
Furthermore, it is requested to simply carry out a desensitizing treatment for the non-image area in the preparation of the printing plate.